Photoelectric cathode and method of producing same



my., 11111` H956 A. H. SOMMER PHOTOELECTRIC CATHODE AND METHDD OF PRODUCNG SAME Filed March 8, 1954 Mw@ M 5.000515 'M00 m-wim hf INI/ENTOR.

United States Patent() Alfred H. Sommer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 8, 1954, Serial No. 414,563

14 Claims. (Cl. 11T- 211) This invention relates to a photosurface and more specifically to a method of forming a photoemissive cathode for use in phototubes, high vacuum photomultiplier tubes or in camera tubes for television.

Antimony films sensitized with cesium have been used as photocathodes in photomultiplier tubes. A semitransparent cesiated antimony photocathode has a spectral response over a range from the ultra violet to around 6400 A. The response of the material is peaked around 4000 A. in the blue with little response in the red. Although this photosurface has fair sensitivity, it is desirable to increase its sensitivity as well as to provide a spectral response extending more toward the red end of the spectrum, Such a spectral response would broaden the use of the photosurface in applications such as pickup tubes for color television, or for flying-spot pickup systems using red luminescing phosphors. t i

lt is thus an object of the invention to provide a photosensitive device having a cathode of improvedsensitivity.

It is another object of the invention to provide an antimony alkali metal photosurface of improved sensitivity.

An additional object of the invention is to provide a photosensitive device having a photocathode containing antimony and having improved sensitivity to red light.

The foregoing and related objects are achieved in accordance with the invention by depositing a lm or layer of antimony on a base support and then sensitizing the surface by a reaction with a plurality of alkali metals. The sensitivity of the photosurface, when formed with a plurality of alkali metals, is much greater than when formed with a single one of the alkali metals. For example, photosurfaces formed of antimony and potassium alone or of antimony and sodium alone, both have a sensitivity in the order of one microampere per lumen. However, a photosurface formed of antimony and both sodium and potassium has been found to have a sensitivity greater than 20 microamperes per lumen. It has also been found that surfaces formed from a plurality of alkali metals usually have a greater spectral response in the red, than those formed with one alkali metal.

The invention is described in greater detail in connection with the accompanying .drawing wherein:

Figure 1 is a sectional View of a photoemissive device having a photocathode in accordance with the invention.

Figure 2 represents the spectral response of different photocathodes formed in accordance with the invention.

Commercial phototubes have been made with a photocathode formed by putting down on a glass end wall of the tube envelope a semitransparent film of antimony and then sensitizing the antimonywith cesium. The photosurface is further activated by baking the tube between 130 C. and 180 C. This type of photosurface has provided a spectral response from the ultra violet to around 6400 A., with little red response as this photosurface is primarily sensitive to blue light.

ln some applications ithas been desirable to'broaden the spectral response of the sensitized antimony photo- ICC surface particularly for use with light sources having emission in the yellow and red regions of the spectrum. Furthermore, it is desirable that the sensitivity of the photosurface be increased to enable it to be used with lowlevel light sources, and to increase the signal-to-noise ratio.

In accordance with the invention, it has been found that the reaction of antimony films with combinations of alkali metals provides photosurfaces having spectral responses which are greater in the yellow and red regions of the spectrum than is produced with antimony and one alkali metal.

Antimony photosurfaces have been made by activating the surface with a single alkali metal other than cesium. Antimony photosurfaces sensitized with any one of the alkali metals potassium, sodium and lithium have provided low sensitivities in the order of one microampere per lumen or less. Antimony films sensitized with rubidium, however, have provided higher sensitivities in the order of l0 microaniperes per lumen.

lt has been found in accordance with the invention that an antimony surface sensitized with potassium sodium and cesium in the manner to be described has not only provided a photosurface with greater sensitivity than a surface formed with a combination of' sodium and potassium but has also produced a much greater spectral response in the red region of the spectrum. Sensitivities well over microamperes per lumen from a tungsten lamp source at a filament color temperature of 2870 K., have been experienced with photosurfaces formed by depositing sodium, potassium and cesium on an antimony film. The sensitivity of this cathode exceeds that of the commercial cesiated antimony photocathode which is known to be the cathode of highest sensitivity at the present time.

Also, in accordance with the invention, an antimony surface sensitized with only potassium and sodium in the manner described below, provides a sensitivity greater than 20 microamperes per lumen, which is in the order of 20 times the sensitivity of an antimony surface sensitized with either potassium or sodium alone. Similarly, lfor example, in accordancel with the invention it has been found that an antimony photosurface sensitized with potassium and lithium and in the manner described below will provide a sensitivity of approximately l0 micro- `amperes per lumen, which is at least 10 times as great as the sensitivities of antimony sensitized with potassium or lithium alone.

Figure l discloses a device, which can be used to form photoemissive surfaces, in accordance with the invention. For example, there is provided an evacuated envelope having a bulb portion 10 and an elongated neck portion 12. Sealed through the closed end of press 14 of the envelope are a pair of leads 16 which extend along the neck into the bulb portion 10. Between the enclosed ends of the leads there is mounted a tungsten filament 18 to which is attached an antimony pellet 20. Attached to the tubular neck portion of envelope 10 are a plurality of side arms containing sensitizing materials for use in forming the photocathode surface of the invention.

The novel photosurface is made in accordance with the following procedure. The antimony pellet fixed to the tungsten filament may be of high grade commercial antimony having a composition substantially 99.88% antimony and only traces of iron, sulphur, arsenic and lead. The tube envelope 10 and 12 is evacuated through an exhaust tubulation 22 which is attached to an exhaust manifold and pumping system not shown. Evacuation of the tube envelope is continued until the pressure used in the envelope is in the order of 10-6 mm. of mercury. A current is passed through the tungsten filament 20 by connecting leads 16 to appropriate sources of current in order to heat the filament to a temperature sufficiently high to evaporate antimony from the pellet 20. The evaporated antimony deposited on the inner surface of the glass bulb is confined to a film 24 of antimony on a preselected portion of the bulb surface, by appropriately masking the other portions of the inner surface of bulb 10.

The evaporation of the antimony is continued until the light transmission through the film 24 is approximately 50% of the light passing through the bulb wall 10 prior to the formation of film 24. Light transmission through the film 24 can be measured in the manner disclosed in the copending application Serial Number 219,997 of l. I. Polkosky filed April 9, 1951, now U. S. Patent 2,676,282. The method is that in which a light beam is passed through the portion of the wall of envelope 10 to be coated with film 24 to fall on a phototube connected to an arnplifying device for continuously measuring the output of the phototube. The light transmission prior to the deposition of film 24 can be considered as 100.

The evaporation of the antimony pellet is continued until the amplifying device registers 50 indicating substantially 50% transmission relative to light transmission through the uncoated glass wall. The bulb portion 10 is supplied with a conducting lead 26 sealed through the glass wall of the bulb to make contact with the antimony film 24.

The lead 26 is connected to the negative terminal of a potential source 25. One of the leads 16 in turn is connected to the positive terminal of the same potential source 25. Potential source provides a potential difference of 100 volts between the antimony film 24 and lead 18 within bulb 10, in order to provide a positive potential on filament 18 relative to the antimony film 24. A microammeter 28 is connected in series in the circuit of lead 16 to measure any electron discharge between film 24 and the positive filament 13, which serves as an anode.

Sodium metal is caused to react with antimony film 24. The sodium can be supplied by providing a mixture 30 of sodium chromate, aluminum and tungsten in a side arm bulb 32 sealed to the tubular envelope neck 12. By heating the sodium chromate mixture, a chemical reaction takes place to release pure sodium metal, which condenses in the bent tubular portion 34 of the side arm. The side arm vessel 32 is disconnected by sealing-off the connecting tube at a point 36.

Bulb portion 10 is raised to a temperature of around 160 C. while the sodium metal in portion 34 is heated slowly to a temperature around 250 C., at which point the sodium vaporizes and passes through the tubular envelope portion 12 into the bulb 10 to react with the antimony film 24. As the sodium reacts with the antimony, film 24 becomes sensitized and emits photoelectrons when exposed to light. This photoemission is detected by the ammeter 2S. The distilling of the sodium metal in tube 34 is continued until the photoemission from the antimony-sodium surface 24 reaches a peak, which will be around one microampere per lumen.

At this point two slightly different procedures may be followed. One is vthat in which heating current is again passed through the tungsten filament 18 to cause additional antimony to be evaporated from pellet 20 and form a second deposit of antimony on the film 24. This evaporation of antimony is continued until practically all photosensitivity of the surface disappears.

Cesium and potassium are made to react with surface 24. This is done by obtaining pure potassium metal from the chemical reaction of potassium chromate, aluminum and tungsten in a second side arm vessel 38. The potassium metal will pass over and condense in the crook of a connecting tubulation 42. Also cesium can be provided by the chemical reaction between cesium chromate and silicon, for example, in a third side arm vessel 44. The cesium metal will also condense in the bent tubing portion 46 of the side arm. After obtaining the pure metals, the side arm vessels 38 and 44 may be respectively removed by sealing-off at the mouth of each vessel. The two metals potassium and cesium are now gently heated to drive the metals from tubing portions 40 and 46, respectively, through the envelope tube 12 into the bulb portion 10, where the metals will react with film 24. During these procedures abulb 10 is maintained between C. and 160 C. The reaction of these metals with film 24 is continued until. the photosensitivity reaches a peak, at which time the two sources of the metals are removed by sealing off the bent tubular portions 40 and 46 respectively. The tube envelope 10 with the photosurface 24 is now baked at 160 C. temperature for a period ot' time and until the photosensitivity reaches another peak. The -bulb 10 is then cooled and the photosensitized film 24 is ready for use.

A second procedure, which has also been found to provide a good photosurface is to cause the cesium and potassium to react with the film 24, immediately following the reaction of the sodium with film 24 and without the intermediate deposit of antimony. The cesium and potassium reaction is continued when photoemission from surface 24 reaches another peak. The two procedures described for making an antimony-sodium-potassiumcesium photosurface are those which have given uniformly excellent photoemission greater than 100 microamperes per lumen.

1t is difcult to guess, and it is not evident what is the function of the several materials in the photoelectric phenomenon. However, the terms lilm, layer, coating, and thelike, used in the specification are not to be construed as necessarily implying physical continuity and homogeneity. It may be that the materials used form into spaced globules or even molecules and that they may also chemically combine with each other to form a discontinuity not possessed by the usual meaning of the terms used.

The antimony-s'odium-potassium-cesium photosurface is one having a higher sensitivity than obtained with any combination of antimony and one alkali metal. The commercial antimony-cesium photosurfaces, as described above, are those averaging around 40 microamperes per lumen, with some going as high as 60 microamperes per lumen. Figure 2 shows a sensitivity curve 52 of an antimony-sodium-potassium-cesium photosurface. The curve indicates a high peak sensitivity in the blue region of the spectrum combined with a relatively high red response. Curve 52 indicates that this type of photosurface has substantially panchromatic sensitivity. Such photosurfaces can be used for color television camera tubes, or black and white television camera tubes.

Other photosurfaces having usable photosensitivities and using other combinations of the alkali metals have been successfully made. For example, one surface having an antimony-potassium-sodium composition is made similar to the procedure described above. For example, an antimony film 24 is put -down on the glass surface of bulb 10 until it has 50% light transmission. Potassium metal is then brought into reactive contact with the antimony surface 24 as described above and until the photoemission reaches a peak sensitivity with the bulb 10 held at 100 C. The potassium source is sealed off and then sodium`metal `is brought into reactive Contact with film 24, with thebulb 10 held at a temperature between 180 C. and 200 C. The sodium metal is distilled until a peak sensitivity is obtained. The sodium source is sealed off and the photosurface 24 with bulb 10 is baked at C.-200 C. until a second sensitivity peak is reached.

Antimony-potassium-sodium photosurfaces of this type have consistently given a photoemission of more than 20 microamperes per lumen,l which is in the order of 20 times greater sensitivity, than when antimony is sensitized with sodium or potassium alone. Figure 2 shows a sensitivity curve 50 for an antimony-sodiumpotassium photosurface. It can be noted that there is a high quantum efficiency in the blue end ofthe spectrum around 4,000 A., together with a cut off at 6,000 and W yresponse between the blue and yellow. A photosurface of this type is of particular va-lue in devices requiring a short wave length cut-ofi?, together with low thermionic emission. The utility of the photosurface would be in such devices as scintillation counters using phophors giving blue light and in which no thermionic emission from the photocathode is desired and a large signal-to-noise ratio is required.

Also, in accordance with the invention, an antimony photosurface may be sensitized with both potassium and lithium to provide a usable photosurface which has a low red response and an increase in total sensitivity as compared with antimony and potassium, or lithium alone. The photosurface of this type may be formed in the manner similar to that described above. For example,

an antimony film 24 is put down on the inner surface of a glass bulb l0 by evaporation of antimony meta-l from the tungsten filament 18. The deposition of film 24 is continued until the light transmission has reached 50%. Potassium metal is next distilled over onto the antimony film from the side tube 44, as described in detail above. The distillation of potassium is continued until the photoemission from film 24 has reached a maximum. Lithium metal is next evaporated onto the cathode film 24 until the photosensitivity passes a maximum value. During this process, the tube bulb 10 is held at a temperature between 180 C. and 220 C. until a second maximum sensitivity is reached.

Antimony photosurfaces, sensitized with both lithium and potassium as described above, have a good blue response combined with a low red response and low t-hermionic emission. The spectrum characteristic curve of such a surface is shown in Figure 2 by curve 56.

The above described methods of forming the several photosensitive surfaces are those which have been performed successfully, as described. However, it has also been found that the order in which the alkali metals are used in sensitizing `the antimony film is not limited to the order set forth above. For example, photosensitive films in which the antimony has been sensitized by either cesium or potassium, first, have been made with excellent sensitivities. It has been found that the apparent order in which the alkali metals are used appears to make no substantial difference, and that the order is merely determined by convenience.

Although the above described photosurfaces are directed to semitransparent films, the invention should not be so limited, as photoemissive surfaces :can be formed as Well with a thicker lm of antimony and on opaque surfaces. The thickness of the antimony is only determined by Whether the exciting light radiations need to pass through the antimony film or not in order to provide photoemission.

What is claimed is:

l. A sensitized electrode comprising a supporting base, a film of antimony on said base, said antimony film including reaction products of antimony with a plurality of alkali metals.

2. A sensitized electrode comprising a supporting base, a film of antimony on said base, said antimony film including reaction products of antimony with sodium and potassium.

3. A sensitized electrode comprising a supporting base, a lm of antimony on said base, said antimony film including reaction products of antimony with sodium, potassium and cesium.

4. A sensitized electrode comprising a supporting base, a film of antimony on said base, said film including small amounts of a plurality of alkali metals.

5. A sensitized electrode comprising a supporting base, a film of antimony on said base, said film including small amounts of sodium, potassium and cesium.

6. A sensitized electrode comprising a supporting base, a film of antimony on said base, said film including small amounts of potassium and lithium.

7. A sensitized electrode comprising a supporting base, a film of antimony 0n said base, said film including small amounts of at least two alkali metals.

8. The method of forming a photosensitive electrode on a support member, said method comprising the steps of forming a film of antimony on said support member, sensitizing said antimony film by depositing thereon a plurality of alkali metals, and subsequent-ly baking said film at an elevated temperature.

9. The method of forming a photosensitive electrode on a support member, said method comprising the steps of, forming a film of antimony on said support member, sensitizing said antimony film by depositing thereon two or more metals selected from the group consisting of sodium, potassium, lithium and cesium, and subsequently baking said film at an elevated temperature.

l0. The method of forming a photosensitive electrode on a support member, said method comprising the steps of forming a film of antimony on said support member, sensitizing said antimony film by depositing thereon sodium, potassium and cesium metals, and baking said film at an elevated temperature.

ll. The method of forming a photosensitive electrode on a support member, said method comprising the steps of forming a semi-transparent film of antimony on said support member, sensitizing said antimony lm by depositing thereon sodium metal, depositing antimony metal on said sensitized antimony lm, again sensitizing said film depositing thereon potassium and cesium metals, and subsequently baking said film at an elevated temperature.

12. The method of forming a photosenstive electrode on a support member, said method comprising the steps of forming a film of antimony having substantially 50% light transmission on said supporting member, sensitizing said antimony film by depositing sodium on said antimony film until photoemission from said film reaches a peak, depositing antimony metal on said sensitized film, sensitizing said film by depositing potassium and cesium on said film until photoemission from said lm reached a second peak, and baking said sensitized film around C. until the photoemission from said film reaches a. third peak.

13. The method of forming a photosensitive electrode on a support member, said method comprising the steps of forming a film of antimony on said supporting member, sensitizing said antimony fihn by depositing thereon sodium until photoemission from said film reaches a peak, depositing on said sensitized lm both cesium and potassium metals, baking said sensitized lm at around 160 C. until the photosensitivity reaches a second peak.

14. The method of forming a photosensitive electrode on a support member, said method comprising the steps of forming a film of antimony on said support member, sensitizing said antimony film by depositing potassium metal on said antimony film until photoemission from the film reaches a maximum, sensitizing said film by depositing lithium metal on said antimony film until photoemission from said film reaches a second maximum value, and baking said sensitized film at a temperature C. and 220 C. until photoemission from said film reached a third maximum.

References Cited in the file of this patent UNITED STATES PATENTS 2,192,418 Sommer Mar. 5, 1940 2,218,340 Maurer Oct. 15, 1940 2,401,736 Janes June 11, 1946 

8. THE METHOD OF FORMING A PHOTOSENSITIVE ELECTRODE ON A SUPPORT MEMBER, SAID METHOD COMPRISING THE STEPS OF FORMING A FILM OF ANTIMONY ON SAID SUPPORT MEMBER, SENSITIZING SAID ANTIMONY FILM BY DEPOSITING THEREON A PLURALITY OF ALKALI METALS, AND SUBSEQUENTLY BAKING SAID FILM AT AN ELEVATED TEMPERATURE. 